Micro-electro-mechanical switch, and methods of making and using it

ABSTRACT

A micro-electro-mechanical (MEMS) switch ( 10, 110 ) has an electrode ( 22, 122 ) covered by a dielectric layer ( 23, 123 ), and has a flexible conductive membrane ( 31, 131 ) which moves between positions spaced from and engaging the dielectric layer. At least one of the membrane and dielectric layer has a textured surface ( 138 ) that engages the other thereof in the actuated position. The textured surface reduces the area of physical contact through which electric charge from the membrane can tunnel into and become trapped within the dielectric layer. This reduces the amount of trapped charge that could act to latch the membrane in its actuated position, which in turn effects a significant increase in the operational lifetime of the switch.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to switches and, more particularly, tomicro-electro-mechanical switches having flexible membranes.

BACKGROUND OF THE INVENTION

One existing type of switch is a radio frequency (RF)micro-electro-mechanical switch (MEMS). This existing type of switch hasa substrate with two spaced and conductive posts thereon. A conductivepart is provided on the substrate between the posts, and is covered by alayer of a dielectric material. A flexible and electrically conductivemembrane extends between the posts, so that a central portion of themembrane is located above the conductive part on the substrate. An RFsignal is applied to one of the conductive part and the membrane.

In the deactuated state of the switch, the membrane is spaced above boththe conductive part and the dielectric layer covering it. In order toactuate the switch, a direct current (DC) bias voltage is appliedbetween the membrane and the conductive part. This bias voltage producescharges on the membrane and the conductive part, and the charges causethe membrane and conductive part to be electrostatically attracted toeach other. This attraction causes the membrane to flex, so that acentral portion thereof moves downwardly until it contacts the top ofthe dielectric layer on the conductive part. This is the actuatedposition of the membrane.

In this actuated state of the switch, the spacing between the membraneand the conductive part is less than in the deactuated state. Therefore,in the actuated state, the capacitive coupling between the membrane andthe conductive part is significantly larger than in the deactuatedstate. Consequently, in the actuated state, the RF signal travelingthrough one of the membrane and conductive part is capacitively coupledsubstantially in its entirety to the other thereof.

In order to deactuate the switch, the DC bias voltage is turned off. Theinherent resilience of the membrane then returns the membrane to itsoriginal position, which represents the deactuated state of the switch.Because the capacitive coupling between the membrane and conductive partis much lower in the deactuated state, the RF signal traveling throughone of the membrane and capacitive part experiences little or nocapacitive coupling to the other thereof.

Although existing switches of this type have been generally adequate fortheir intended purposes, they have not been satisfactory in allrespects. One problem is that, when the membrane is contacting thedielectric layer in the actuated state of the switch, electric chargefrom the membrane can tunnel into and become trapped in the dielectriclayer. As a result, and due to long recombination times in thedielectric, the amount of this trapped charge in the dielectricincreases progressively over time.

The progressively increasing amount of trapped charge exerts aprogressively increasing attractive force on the membrane. When themembrane is in its actuated position, this attractive force tends toresist movement of the membrane away from its actuated position towardits deactuated position. The amount of trapped charge can eventuallyincrease to the point where the attractive force exerted on the membraneby the trapped charge is in excess of the inherent resilient force ofthe membrane which is urging the membrane to return to its deactuatedposition. As a result, the membrane becomes trapped in its actuatedposition, and the switch is no longer capable of carrying out aswitching function. This is considered a failure of the switch, and isassociated with an undesirably short operational lifetime for theswitch. In this regard, an RF MEMS switch of this type should be capableof trillions of switching cycles before a failure occurs due to fatiguein the metal of the membrane, but trapped charge in the dielectricusually results in failure after only millions of switching cycles.

There are many applications in which a switching function can beimplemented using either a field effect transistor (FET) switch or an RFMEMS switch. However, due in significant part to the dielectric chargingproblem discussed above, the operational lifetime of existing MEMSswitches is significantly shorter than the operational lifetime ofcommercially available FET switches. Consequently, FET switches arecurrently favored over MEMS switches for these applications.

Prior attempts have been made to solve the dielectric charging problem.One approach was to change the properties of the dielectric material soas to modify the extent to which the dielectric material is “leaky”. Forexample, by adding more silicon to silicon nitride used for thedielectric material, the conductivity of the dielectric materialincreases, and then it becomes easier for the trapped charges torecombine in a manner which neutralizes them. However, this approachalso increases the power consumption of the MEMS switch, and has notbeen shown to provide a significant increase in its operationallifetime.

Another prior approach to the dielectric charging problem is to alterthe waveform used for the DC bias voltage. For example, lowering theactuation voltage reduces the amount of charge which tunnels into thedielectric material, and thus reduces the rate at which the amount oftrapped charge within the dielectric material can increase. Further, theslope of the release waveform can be decreased, so as to give thetrapped charges more time to recombine. These types of changes to theactuation waveform can produce a significant increase in the operationallifetime of a MEMS switch. However, they also significantly increase theswitching time of the switch, for example by a factor of approximately20, which in turn renders such a MEMS switch highly undesirable for manyapplications that involve high switching speeds.

In the design of MEMS switches, a traditional design goal has been totry to maximize the capacitance ratio of the switch, which is the ratioof the capacitance between the membrane and conductive part in theactuated state to the corresponding capacitance in the deactuated state.In an effort to maximize the capacitance in the actuated state,pre-existing MEMS switch designs attempt to position the membrane asclose as possible to the conductive part in the actuated state of theswitch, which in turn means that the dielectric layer separating themneeds to be relatively thin. Consequently, the surfaces of the membraneand dielectric layer which engage each other have traditionally beenintentionally polished or otherwise fabricated to make them as smooth aspossible, so that both surfaces have their entire areas in directphysical contact with each other when the membrane is in its actuatedposition, thereby positioning as much of the membrane as possible invery close proximity to the conductive part.

SUMMARY OF THE INVENTION

From the foregoing, it may be appreciated that a need has arisen for amethod and apparatus for making and operating a switch of the typehaving a flexible membrane, in a manner so that the switch has asignificantly increased operational lifetime. According to the presentinvention, a method and apparatus are provided to address this need.

More specifically, according to one form of the invention, a switchincludes a base having a first section which includes an electricallyconductive part, and also includes a membrane having first and secondends supported at spaced locations on the base, and having between theends a second section which includes an electrically conductive portion.The membrane is capable of resiliently flexing so as to move betweenfirst and second positions, the conductive part and the conductiveportion being physically closer in the second position than in the firstposition. One of the first and second sections has a textured surface,and the other thereof has a further surface which faces the texturedsurface, the textured surface having mutually exclusive first and secondportions which are respectively in physical contact with and free ofphysical contact with the further surface when the membrane is in thesecond position. The first portion of the textured surface has an areawhich is substantially less than a total area of the textured surface.

According to a different form of the invention, a switching method usesa switch that includes a base having a first section with anelectrically conductive part, and that includes a membrane having firstand second ends supported at spaced locations on the base, and havingbetween the ends a second section which includes an electricallyconductive portion, where one of the first and second sections has atextured surface and the other thereof has a further surface which facesthe textured surface. The method includes: responding to an appliedvoltage between the conductive part and the conductive portion byresiliently flexing the membrane so that the conductive part movescloser to the conductive portion as the membrane moves from a firstposition to a second position. This includes causing mutually exclusivefirst and second portions of the textured surface to respectively be inphysical contact with and free of physical contact with the furthersurface when the membrane is in the second position, the first portionof the textured surface having an area which is substantially less thana total area of the textured surface.

According to still another form of the invention, a method offabricating a switch includes: forming on a base a first section whichincludes an electrically conductive part; forming a resiliently flexiblemembrane having first and second ends engaging spaced portions of thebase disposed on opposite sides of the first section, and having betweenthe ends a second section which includes an electrically conductiveportion, the membrane being capable of resiliently flexing so as to movebetween first and second positions so that the conductive part and theconductive portion are physically closer in the second position than inthe first position; and forming on one of the first and second sectionsa textured surface and on the other thereof a further surfaces whichfaces the textured surface, the textured surface having mutuallyexclusive first and second portions which are respectively in physicalcontact with and free of physical contact with the further surface whenthe membrane is in the second position, the first portion of thetextured surface having an area which is substantially less than a totalarea of the textured surface.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be realized fromthe detailed description which follows, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a diagrammatic fragmentary sectional side view of an apparatuswhich includes a micro-electro-mechanical switch (MEMS) that embodiesaspects of the present invention;

FIG. 2 is a diagrammatic fragmentary sectional side view similar to FIG.1, but showing the switch of FIG. 1 in a different operational state;

FIG. 3 is a diagrammatic fragmentary sectional side view showing aportion of the switch of FIG. 1, at an intermediate point during itsfabrication;

FIG. 4 is a diagrammatic fragmentary sectional side view similar to FIG.3, but showing part of the switch of FIG. 1 at a later point duringfabrication of the switch;

FIG. 5 is a diagrammatic fragmentary sectional side view similar to FIG.1, but showing a micro-electro-mechanical switch (MEMS) which is analternative embodiment of the switch of FIG. 1;

FIG. 6 is a diagrammatic fragmentary sectional side view similar to FIG.5, but showing the switch of FIG. 5 in a different operational state;

FIG. 7 is a diagrammatic fragmentary sectional side view showing aportion of the switch of FIG. 5, at an intermediate point during itsfabrication;

FIG. 8 is a diagrammatic fragmentary sectional side view similar to FIG.7, but showing part of the switch of FIG. 5 at a later point duringfabrication of the switch; and

FIG. 9 is a diagrammatic fragmentary sectional side view similar to FIG.8, but showing the switch of FIG. 5 at still a later point during itsfabrication.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagrammatic fragmentary sectional side view of an apparatuswhich includes a micro-electro-mechanical switch (MEMS) 10, the switch10 embodying aspects of the present invention. The drawings, includingFIG. 1, are diagrammatic and not to scale, in order to present theswitch 10 in a manner which facilitates a clear understanding of thepresent invention.

With reference to FIG. 1, the switch 10 includes a silicon semiconductorsubstrate 13 having on an upper side thereof an oxide layer 14. Althoughthe substrate 13 is a made of silicon in this disclosed embodiment, itcould alternatively be made of some other suitable material, such asgallium arsenide (GaAs), or a suitable alumina. Similarly, the oxidelayer 14 is silicon dioxide in this disclosed embodiment, but couldalternatively be some other suitable material.

Two posts 17 and 18 are provided at spaced locations on the oxide layer14, and are each made of a conductive material. In this embodiment theposts are made of gold, but they could alternatively be made of someother suitable conductive material. A plurality of diagrammaticallydepicted nodules 21 are provided on the upper surface of the oxide layer14, at a location intermediate the posts 17 and 18, in order to create adegree of roughness or texture on this part of the top surface of theoxide layer 14. In the embodiment of FIG. 1, the nodules 21 are made ofsilicon titanium (SiTi), and have a silicon to titanium ratio of about5:1. However, the nodules 21 could alternatively be made from some othersuitable material. In the embodiment of FIG. 1, the nodules 21 have avertical height of approximately 100 to 500 nm, but could alternativelyhave some other suitable height.

Instead of using the silicon substrate with nodules thereon, it wouldalternatively be possible to omit the nodules and to use a substrate ofsome other material, such as alumina, which has a top surface inherentlyrougher than the top surface of silicon. The electrode and dielectriclayer would then conform to the rough surface on top of the alumina inorder to create a textured surface on top of the dielectric layer.Although alumina has previously been used in pre-existing MEMS switches,it has customarily been highly polished so as to eliminate anysignificant roughness, in an effort to maximize the capacitance ratio,as discussed earlier.

An electrically conductive electrode 22 serves as a transmission line,and is elongated in a direction perpendicular to the plane of FIG. 1. Inthe embodiment of FIG. 1, the electrode 22 is made of gold, but it couldalternatively be made from some other suitable material. A centralportion of the electrode 22, which is visible in FIG. 1, isapproximately 300 to 400 nm thick, and extends over the nodules 21 andthe adjacent portions of the top surface of the oxide layer 14.Consequently, in view of the vertical height of the nodules 21, theelectrode 22 conforms generally to the shape of an upwardly facingsurface defined by surface portions on top of the nodules 21 and oxidelayer 14. Thus, the top surface of the central portion of the electrode22 has a degree of roughness or texture.

This central portion of the electrode 22 is covered by a dielectriclayer 23. In the disclosed embodiment, the dielectric layer 23 is madeof silicon nitride, and has a thickness of approximately 100 to 300 nm.The dielectric layer 23 conforms in shape to the top surface of theelectrode 22, and thus the top surface of the electrode 23 has a degreeof roughness or texture. The substrate 13, oxide layer 14, posts 17-18,nodules 21, electrode 22 and dielectric layer 23 can be collectivelyreferred to as a base portion of the switch 10.

A conductive membrane 31 extends between the upper ends of the posts 17and 18. In the disclosed embodiment, the membrane 31 is made of a knownaluminum alloy, and in fact could be made of any suitable material thatis commonly used to fabricate membranes in MEMS switches. The membrane31 has ends 32 and 33, which are each fixedly supported on the topportion of a respective one of the posts 17 and 18. The membrane 31 has,between its ends 32 and 33, a central portion 36 which is disposeddirectly above the electrode 22 and the dielectric layer 23.

The membrane 31 is approximately planar in the view of FIG. 1, but iscapable of flexing so that its central portion 36 moves downwardly untilit contacts the textured top surface of the dielectric layer 23. Thisflexed position is shown in FIG. 2, which is a diagrammatic fragmentarysectional side view showing the same structure as FIG. 1, but in adifferent operational state.

During operational use of the switch 10, a radio frequency (RF) signalhaving a frequency in the range of approximately 300 MHz to 90 GHz iscaused to travel through one of the membrane 31 and the electrode 22.More specifically, the RF signal may be traveling from the post 17through the membrane 31 to the post 18. Alternatively, the RF signal maybe traveling through the electrode 22 in a direction perpendicular tothe plane of FIGS. 1 and 2.

Actuation of the switch 10 is carried out under control of a directcurrent (DC) bias voltage, which is applied between the membrane 31 andthe electrode 22 by a control circuit of a type which is well-known inthe art, and which is therefore not illustrated and described. This biasvoltage can also be referred to as a pull-in voltage (V_(p)). When thebias voltage is not applied to the switch 10, the membrane 31 is in theposition shown in FIG. 1. As discussed above, an RF signal will bepassing through one of the membrane 31 and the electrode 22. Forconvenience in the discussion which follows, it will be assumed that theRF signal is passing through the electrode 22. When the membrane 31 isin the deactuated position of FIG. 1, the RF signal traveling throughthe electrode 22 will pass through the switch 10 and continue travelingthrough the electrode 22, with no significant coupling of this RF signalfrom the electrode 22 over to the membrane 31.

In order to actuate the switch 10, a DC bias voltage (pull-in voltageV_(p)) is applied between the electrode 22 and the membrane 31. Thisbias voltage produces charges on the membrane 31 and on the electrode22, which in turn produce an electrostatic attractive force that urgesthe central portion 36 of the membrane 31 toward the electrode 22. Thisattractive force causes the membrane 31 to flex downwardly, so that itscentral portion 36 moves toward the electrode 22. The membrane 31 flexesuntil its central portion 36 engages the textured top surface of thedielectric layer 23, as shown in FIG. 2. This is the actuated positionof the membrane. In this position, the capactive coupling between theelectrode 22 and the central portion 36 of the membrane 31 isapproximately 100 times greater than when the membrane 31 is in thedeactuated position shown in FIG. 1. Consequently, the RF signaltraveling through the electrode 22 will be coupled substantially in itsentirety from the electrode 22 over into the membrane 31, where it willtend to have two components that travel away from the central portion 36of the membrane in opposite directions, toward each of the posts 17 and18. Alternatively, if the RF signal had been traveling through themembrane 31 from the post 17 to the post 18, the RF signal would havebeen coupled substantially in its entirety from the central portion 36of the membrane over to the electrode 22, where it would tend to havetwo components that travel away from the switch 10 in respectiveopposite directions through the electrode 22.

Once the membrane 31 has reached the actuated position shown in FIG. 2,the not-illustrated control circuit may optionally reduce the DC biasvoltage (pull-in voltage V_(p)) to a standby or hold value. The standbyor hold value is less than the voltage that was needed to initiatedownward movement of the membrane 31 from the position shown in FIG. 1,but is sufficient to maintain the membrane 31 in the actuated positionof FIG. 2, once the membrane has reached this actuated position.

While the membrane 31 is in the actuated position of FIG. 2, thetextured top surface of the dielectric layer 23 causes the actualphysical contact between the dielectric layer 23 and electrode 31 to belimited to a number of spaced contact regions that are each relativelysmall in area. In other words, the total area of physical contactbetween the dielectric layer 23 and the membrane 31 is substantiallyless than would be the case if the dielectric 23 had a smooth and flattop surface which, in its entirety, was in physical contact with thesmooth and approximately flat underside of the central portion 36 of themembrane 31. Since the operative coupling between the membrane 31 andelectrode 22 involves capactive coupling, rather than direct physicalcontact, reducing the total amount of direct physical contact betweenthem does not have a significant effect on the operation of the switch10.

When a textured surface of the type shown in FIG. 1 is used, thecapacitance between the membrane 31 and the electrode 22 in the actuatedstate of FIG. 2 may be slightly less that it would be if the dielectriclayer 23 had a traditional flat top surface. Consequently, the ratio ofthe capacitance for the actuated state of FIG. 2 to the capacitance forthe deactuated state of FIG. 1 may be slightly less than when thedielectric layer 23 has a traditional flat surface. However, this smallreduction in the capacitance ratio is negligible at higher frequencies,and any minor disadvantage is outweighed by the fact that a significantadvantage is obtained from use of the textured surface. In particular,by using the textured surface to reduce the total area of actualphysical contact between the membrane 31 and the dielectric layer 23,there is less total area of physical contact through which electriccharge from the membrane 31 can pass, and this in turn reduces theamount of charge that can tunnel into and become trapped in thedielectric layer 23. This means that the rate at which trapped chargecan build up in the dielectric layer 23 is substantially lower for theswitch of FIGS. 1-2 than for pre-existing switches.

As a result, it takes much longer for the switch 10 to reach a statewhere the amount of trapped charge in the dielectric layer can attractthe membrane 31 with a force sufficiently large to prevent the switch 10from deactuating when the DC bias voltage (pull-in voltage V_(p)) isterminated. Therefore, the effective operational lifetime of the switch10 is substantially longer than for pre-existing switches which do nothave the textured surface. In fact, the textured surface extends theoperational lifetime of the switch so much that the limiting factor onoperational lifetime becomes physical fatigue and/or failure of themembrane 31, rather than trapping of the membrane 31 due to trappedcharges in the dielectric layer 23. In this regard, switch 10 will havean operational lifetime that can be 1,000 to 1,000,000,000 times longerthat the operational lifetime of comparable pre-existing switches thatlack the textured surface.

A secondary advantage of the textured surface is that, by reducing thetotal area of physical contact between the membrane 31 and thedielectric layer 23, there is a reduction in Van Der Walls forces whichtend to cause attraction between the membrane 31 and dielectric layer23, and which thus resist movement of the membrane 31 away from thedielectric layer 23.

In order to deactivate the switch 10, the not-illustrated controlcircuit terminates the DC bias voltage (pull-in voltage V_(p)) that isbeing applied between the membrane 21 and the electrode 22. The inherentresilience of the flexible membrane 31 produces a relatively strongrestoring force, which causes the central portion 36 of the membrane tomove upwardly away from the dielectric layer 23 and the electrode 22,until the membrane reaches the position shown in FIG. 1.

FIG. 3 is a diagrammatic fragmentary sectional side view of part of theswitch 10 of FIG. 1, showing the switch at an intermediate point duringits fabrication. Fabrication of the switch 10 begins with provision ofthe silicon substrate 13, and then the silicon oxide layer 14 is grownon the substrate 13 using known techniques.

Next, a layer 71 of photoresist is applied over the oxide layer 14. Thephotoresist layer 71 is then patterned and etched using knowntechniques, so as to define through the layer 71 an opening 72 in theregion where the electrode 22 (FIG. 1) will eventually be formed. Next,a layer 74 of an aluminum alloy is sputtered over the layer 71, so thata portion of the layer 74 engages the oxide layer 14 within the opening72 through the photoresist 71. In the disclosed embodiment, the aluminumalloy layer 71 is approximately 300 nm thick, and contains approximately98.8% aluminum (Al), 1% silicon (Si), and 0.2% titanium (Ti). The layer74 is then wet etched, in order to remove the aluminum through aluminumleach. The aluminum leach that occurs during the wet etch does notremove the silicon and titanium, thereby leaving the SiTi nodules 21(FIG. 1), which are approximately 100 to 500 nm in vertical height.Next, the photoresist layer 71 is removed in a known manner. Any SiTinodules present on the layer 71 itself are removed with the layer 71,thereby leaving only the SiTi nodules 21 located directly on the oxidelayer 14, as shown diagrammatically in FIG. 1.

FIG. 4 is a diagrammatic fragmentary sectional side view similar to FIG.3, but showing part of the switch at a later point during itsfabrication. With reference to FIG. 4, the next step in the fabricationof the switch is to form the electrode 22 over the nodules 21 and theoxide layer 14, for example by depositing a layer of gold and thencarrying out a patterned etch. After that, the dielectric layer 23 isformed, for example by depositing a layer of silicon nitride and thencarrying out a patterned etch.

Next, the posts 17 and 18 are formed, by depositing a layer of gold, andthen carrying out a patterned etch so to leave just the posts 17-18.Then, a spacer layer 81 is formed over the oxide layer 14, dielectriclayer 23 and posts 17-18. The spacer layer 76 is a photoresist materialof a type known to persons skilled in the art. The spacer layer 76 ispatterned, etched, and/or planarized, in order to give it a desiredshape and thickness. After that, a layer of a known aluminum alloy isdeposited over the spacer layer 76, the posts 17-18 and the oxide layer14, and is patterned and etched in order form the membrane 31. At thispoint, the structure has the configuration which is shown in FIG. 4.

Next, an etch procedure referred to as a membrane release etch iscarried out, in order to remove the spacer layer 76 in its entirety. Themembrane release etch may, for example, be a plasma etch of a knowntype, or any other suitable etch that will attack the material of thephotoresist forming the spacer layer 76. This etch leaves the membrane31 suspended on the posts 17-18 by its ends 32 and 33. This is thefinished configuration of the switch 10, which is shown in FIG. 1.

FIG. 5 is a diagrammatic fragmentary sectional side view of an apparatusthat includes a micro-electro-mechanical switch (MEMS) 110, which is analternative embodiment of the switch 10 of FIG. 1. Except fordifferences which are described below, the switch 110 is generallysimilar the switch 10, and identical parts are identified by the samereference numerals.

The switch 110 includes a substrate 13, oxide layer 14, and posts 17 and18, which are equivalent to their counterparts in the embodiment of FIG.1. An electrode 122 is provided on the oxide layer 14 intermediate theposts 17-18, and is covered by a dielectric layer 123. It will be notedthat the SiTi nodules 21 in FIG. 1, are omitted from the switch 110 ofFIG. 5. Consequently, the electrode 122 is disposed directly on a flattop surface of the oxide layer 14, and the electrode 122 thus has anapproximately flat upper surface. A portion of the top surface of thedielectric layer 123 which is located directly above the electrode 122is also flat, rather than textured. Aside from this, the electrode 122and dielectric layer 123 are generally equivalent to the electrode 22and dielectric layer 23 in the switch 10 of FIG. 1.

In FIG. 5, an electrically conductive membrane 131 extends between theupper ends of the posts 17 and 18, and has ends 132 and 133 which areeach fixedly supported on top of a respective one of the posts 17 and18. The membrane 131 is generally equivalent to the membrane 31 in theswitch 10 of FIG. 1, except that the membrane 131 has a textured surface138 on the underside of a central portion 136 thereof. The texturedsurface 138 includes several projections or bosses that projectdownwardly toward the electrode 122 and the dielectric layer 123, andwhich are spaced from each other.

The membrane 131 can resiliently flex from the deactuated position shownin FIG. 5 to an actuated position. In this regard, FIG. 6 is adiagrammatic fragmentary sectional side view showing the switch 110 ofFIG. 5, but with the membrane 131 in its actuated position. In thisactuated position, the dielectric layer 123 engages only spaced portionsof the textured surface 138 which are located at the ends of the bosses.Consequently, the total area of actual physical contact between themembrane 131 and the dielectric layer 123 is less than would be the caseif the flat surface of the dielectric layer was engaging a flat surfaceon the membrane.

The switch 110 of FIGS. 5-6 operates in a manner similar to theoperation of the switch 10 of FIGS. 1-2. Accordingly, it is believed tobe unnecessary to provide a separate detailed explanation of theoperation of the switch 110.

FIG. 7 is a diagrammatic fragmentary sectional side view of part of theswitch 110 of FIG. 5, at an intermediate stage during fabrication of theswitch 110. With reference to FIG. 7, fabrication of the switch 110begins with provision of the silicon substrate 13, and then the oxidelayer 14 is grown on the silicon substrate 13. After that, the electrode142 is formed on the oxide layer 14, for example by depositing a layerof gold and then carrying out a patterned etch. Next, the dielectriclayer 123 is formed, for example by depositing a layer of siliconnitride, and then carrying out a patterned etch.

Next, the posts 17 and 18 are formed, for example by depositing a layerof gold and then carrying out a patterned etch that removes unwantedmaterial, so as to leave just the posts 17 and 18. Then, a spacer layer171 is formed over the oxide layer 14, the dielectric layer 123, and theposts 17-18. The spacer layer 176 is a photoresist material of a knowntype, which is patterned and etched in order to give it a desired shape.The resulting structure may be planarized, so that the top surfaces ofthe posts 17 and 18 are substantially flush with the top surface of thespacer layer 171.

A mask 176 is then placed over the partially completed device. In FIG.7, the mask 176 is shown resting on the top surfaces of the spacer layer171 and the posts 17-18, but the mask 176 may alternatively be spacedslightly above these surfaces. The mask 176 includes a glass layer 177which is transparent to ultraviolet radiation, and a chrome layer 178which is provided on the underside of the glass layer 177. The chromelayer 178 is non-transmissive to ultraviolet radiation. The chrome layer178 has, in a central portion thereof immediately above electrode 122and the dielectric layer 123, a cluster of spaced openings, one of whichis identified by reference numeral 183. In the disclosed embodiment, theopenings 183 are circular and each have a diameter in the range ofapproximately 100 nm to 500 nm, but they could alternatively have someother suitable shape or size. Using alignment techniques known to thoseskilled in the art, the mask 176 is accurately positioned with respectto the structure being fabricated, so that the cluster of openings 183is accurately centered above the electrode 122 and the dielectric layer123.

Next, the structure shown in FIG. 7 is exposed to ultraviolet radiationfor a predetermined time interval, as indicted diagrammatically byarrows 184. Radiation which impinges on the chrome layer 178 will beeither reflected or absorbed. The remaining radiation will pass throughthe openings 183, and will “expose” spaced regions of the photoresistmaterial located adjacent the top surface of the spacer layer 171.

The mask 176 is then removed, and the spacer layer 171 is etched usingknown techniques, so as to remove material of the spacer layer 171 whichhas been exposed to light. The result is spaced recesses or dimples inthe top surface of the spacer layer 171. In this regard, FIG. 8 is adiagrammatic fragmentary sectional side view similar to FIG. 7, butshowing part of the switch 110 at a later stage in its fabrication. InFIG. 8, reference numeral 186 designates one of the recesses that arecreated in the top surface of the spacer layer 171 by the etchprocedure. The strength and duration of the etch procedure are selectedto give the recesses 186 a desired depth. In the disclosed embodiment,the recesses 186 have a depth of approximately 100 nm, but it would bepossible for the recesses to alternatively have a larger or smallerdepth.

Next, the membrane 131 is formed by depositing a layer of a knownaluminum alloy over the spacer layer 171, the posts 17-18, and the oxidelayer 14. This layer of aluminum alloy is then patterned and etched, inorder to form the membrane 131. The central portion 136 of the membrane131 conforms in shape to the top surface of the spacer layer 171,including the recesses 186 therein. Thus, the portion of the top surfacehaving the recesses 186 creates the textured surface 138 on theunderside of the central portion 136 of the membrane 131.

Thereafter, a known etch procedure referred to as a membrane releaseetch is carried out, in order to remove the spacer layer 171 in itsentirety. This etch leaves the membrane 131 suspended on the posts 17-18by its ends 132 and 133. This is the finished configuration of theswitch 110, which is shown in FIG. 5.

The present invention provides a number of technical advantages. Onesuch technical advantage is that a MEMS switch embodying the presentinvention has a useful lifetime which is several orders of magnitudebetter than pre-existing MEMS switches. In this regard, the provision ofa textured surface on at least one of the membrane and dielectric layerreduces the total area of physical contact between the membrane anddielectric layer. This in turn reduces the amount of charge from themembrane which can tunnel into and become trapped in the dielectriclayer, thereby decreasing the voltage which charge trapped in thedielectric can exert on the membrane in a manner that could eventuallylatch the membrane in its actuated position.

Due to the textured surface, the operational lifetime of the switchbegins to approach the operational lifetime of certain field effecttransistor (FET) switches, thereby permitting a MEMS switch whichembodies the invention to compete commercially for use in applicationsthat traditionally were restricted to FET switches. A further advantageis that the textured surface tends to reduce the extent to which Van DerWalls forces and/or contamination can resist movement of the membraneaway from the dielectric layer when the switch is deactuated.

Although selected embodiments have been illustrated and described indetail, it will be understood that various substitutions and alterationsare possible without departing from the spirit and scope of the presentinvention, as defined by the following claims.

What is claim is:
 1. An apparatus comprising a micro-electro-mechanicalswitch which includes: a base having a first section which includes anelectrically conductive part; a membrane having first and second endssupported at spaced locations on said base, and having between said endsa second section which includes an electrically conductive portion, saidmembrane being capable of resiliently flexing so as to move betweenfirst and second positions, said conductive part and said conductiveportion being physically closer in said second position than in saidfirst position, one of said first and second sections having a texturedsurface and the other thereof having a further surface which faces saidtextured surface, said textured surface having mutually exclusive firstand second portions which are respectively in physical contact with andfree of physical contact with said further surface when said membrane isin said second position, said first portion of said textured surfacehaving an area which is substantially less than a total area of saidtextured surface; and wherein one of said first and second sectionsincludes a dielectric material having thereon one of said textured andfurther surfaces, said one of said textured and further surfaces beingformed of the dielectric material, the other of said textured andfurther surfaces being provided on one of said conductive part and saidconductive portion.
 2. An apparatus according to claim 1, wherein saidfirst portion of said textured surface is defined by a plurality ofseparate regions of said textured surface which are spaced from eachother.
 3. An apparatus comprising a micro-electro-mechanical switchwhich includes: a base having a first section which includes anelectrically conductive part; a membrane having first and second endssupported at spaced locations on said base, and having between said endsa second section which includes an electrically conductive portion, saidmembrane being capable of resiliently flexing so as to move betweenfirst and second positions, said conductive part and said conductiveportion being physically closer in said second position than in saidfirst position, one of said first and second sections having a texturedsurface and the other thereof having a further surface which faces saidtextured surface, said textured surface having mutually exclusive firstand second portions which are respectively in physical contact with andfree of physical contact with said further surface when said membrane isin said second position, said first portion of said textured surfacehaving an area which is substantially less than a total area of saidtextured surface; wherein said second section includes a dielectricmaterial having said textured surface on a side thereof facing saidfirst section, said textured surface being formed of the dielectricmaterial, said conductive portion being disposed on an opposite side ofsaid dielectric material from said first section; and wherein saidfurther surface is provided on said conductive part of said membrane. 4.An apparatus according to claim 3, wherein said conductive portion has atextured surface on a side thereof nearest said first section; andwherein said dielectric material is a dielectric layer provided over andconforming a shape of said textured surface on said conductive portionso as to define on said dielectric layer said textured surface havingsaid first and second portions.
 5. An apparatus according to claim 3,wherein said base includes a portion having a textured surface on a sidethereof nearest said first section; wherein said conductive portion is aconductive layer provided over and conforming to a shape of saidtextured surface on said portion of said base so as to define on a sideof said conductive layer facing said first section a textured surface;and wherein said dielectric material is a dielectric layer provided overand conforming to said textured surface on said conductive part so as todefine on said dielectric layer said textured surface having said firstand second portions.
 6. An apparatus according to claim 5, wherein saidportion of said base includes a base surface which faces said firstsection, and includes a plurality of nodules fixedly provided on saidbase surface, said textured surface on said portion of said base beingdefined by surface portions disposed on said nodules and said layer. 7.An apparatus comprising a micro-electro-mechanical switch whichincludes: a base having a first section which includes an electricallyconductive part; a membrane having first and second ends supported atspaced locations on said base, and having between said ends a secondsection which includes an electrically conductive portion, said membranebeing capable of resiliently flexing so as to move between first andsecond positions, said conductive part and said conductive portion beingphysically closer in said second position than in said first position,one of said first and second sections having a textured surface and theother thereof having a further surface which faces said texturedsurface, said textured surface having mutually exclusive first andsecond portions which are respectively in physical contact with and freeof physical contact with said further surface when said membrane is insaid second position, said first portion of said textured surface havingan area which is substantially less than a total area of said texturedsurface; wherein said second section includes a dielectric materialhaving said further surface on a side thereof facing said first section,said further surface being formed of the dielectric material, saidconductive portion being disposed on an opposite side of said dielectricmaterial from said first section; and wherein said textured surface isprovided on said conductive part.
 8. An apparatus according to claim 3,wherein said first portion of said textured surface is defined by aplurality of separate regions of said textured surface which are spacedfrom each other.
 9. An apparatus according to claim 7, wherein saidfirst portion of said textured surface is defined by a plurality ofseparate regions of said textured surface which are spaced from eachother.